Circuit Modifications

The circuit was re-drawn primarily for reference, in order to maintain a service manual for the equipment. The redrawn circuit also is an improvement in legibility and allowed some simulations to be performed on various parts of the circuit to check certain component substitutions and the calibration process.

Since deciding to build the unit using rotary switches for ratio and metering, another small modification was made to the original circuit. This modification adds two extra compression ratios (6:1 and 10:1) and makes use of a 6-position rotary switch for the ratio selector.

As a result a new switch module was designed. Instead of producing two PCB's (one for the ratio and one for the metering switch) a single PCB was designed with the switches spaced according to the panel layout. This was primarily to allow room for the extra resistors required.

The power supply in the original circuit was not used. The use of 78xx regulators with audio circuits can be problematic, since they are prone to noise generation. Instead the lower noise adjustable LM317 and LM337 devices were used on a power supply PCB (PSU-8). Component changes allowed the outputs to be adjusted to +30V and -10V easily.

The original FETs (2N5457) were kept, since they were easily obtainable. To simplify the tracking calibration between the signal preamp section and the metering circuit (each with its own FET), a number of devices were purchased and, using a simple test jig, matching was performed to find a suitable pair of devices.